iio: add vcnl4000 combined ALS and proximity sensor

minimal driver, no IR current control and proximity/event
handling yet

v5:
* checkpatch warnings
* increase msleep() to 20 ms when waiting for data ready as
  measurement/conversion can take up to 100 ms, 1 ms is too short

v4 (address comments by Jonathan Cameron)
* remove SENSORS_ prefix in Kconfig
* change from IIO_INTENSITY to IIO_LIGHT
* move from staging

v3 (address comments by Shubhrajyoti Datta)
* cleanup Kconfig entry
* call I2C read/write functions directly

v2 (address comments by Lars-Peter Clausen and Jonathan Cameron)
* unify code for reading PS and AL data into
  parameterized _measure() function
* limit wait for data to become ready within 20 tries
* drop IIO_LIGHT channel, add SCALE to IIO_INTENSITY
* drop extra string arguments used for logging purpose only

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/iio/light/Kconfig b/drivers/iio/light/Kconfig
index db5618e..f3ea90d 100644
--- a/drivers/iio/light/Kconfig
+++ b/drivers/iio/light/Kconfig
@@ -19,4 +19,15 @@
 	  light zone through sysfs. A threshold event can be generated on zone
 	  changes. The ALS-control output values can be set per zone for the
 	  three current output channels.
+
+config VCNL4000
+	tristate "VCNL4000 combined ALS and proximity sensor"
+	depends on I2C
+	help
+	 Say Y here if you want to build a driver for the Vishay VCNL4000
+	 combined ambient light and proximity sensor.
+
+	 To compile this driver as a module, choose M here: the
+	 module will be called vcnl4000.
+
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